Conventional devices for producing hot water comprise a (generally tubular) exchanger wherein the water to be heated circulates in order to transfer calories through an exchange wall. The main drawback of these exchange circuit devices lies in the poor heat transfer efficiency level.
A new type of device for producing hot water, referred to as direct contact device, has appeared in the past ten years, wherein the water to be heated is sprayed and flows through the combustion chamber so that the exchange occurs directly without any interposed exchange wall; this type of direct contact device is characterized in that the combustion chamber is wide open so as to allow direct contact between the flame and the sprayed water. In this new type of device, a layer of transfer nodules is most often arranged on the path of the smoke gases in order to ensure more complete heat transfer. An excellent transfer efficiency is thus obtained, that can be illustrated by the outlet temperature of the smoke gases which exceeds the temperature of the hot water obtained by a few degrees only (of the order of 5 to 10.degree. C.). External hot gases are sometimes recovered and used as an additional heat input.
These direct contact devices are described in particular in the following patents U.S. Pat. No. 4,275,708, U.S. Pat. No. 4,574,775, U.S. Pat. No. 5,293,861, EP-0,082,139. It can be noted that this recent hot water production technique by direct contact had already been proposed in a very ancient U.S. patent (U.S. Pat. No. 884,223 delivered on Apr. 7, 1908) which describes a gas ramp C arranged at the base of an open combustion chamber E above which the water to be heated trickles.
Direct contact devices such as those described for example in patents GB-2,129,916 and EP-0,387,983 allow to reach remarkable transfer efficiency levels but they have several drawbacks. The most serious one, which considerably limits the development thereof, lies in the very high carbon monoxide ratio observed in the smoke gases produced by this type of device. This ratio, generally of the order of 500 to 700 ppm, remains, in the best case, above 150 to 200 ppm, which exceeds the ratios authorized by most standards (housing: ratio below 100 ppm in France). Another drawback of these devices comes from the very high temperature of the walls delimiting the combustion chamber (temperature of the order of 800.degree. to 900.degree. close to that of the flame): these walls, situated in an air/sprayed water/steam atmosphere, undergo great corrosion stresses; they can be equipped with cooling circuits, but this considerably increases the complexity and the cost of the device.